U.S. Pat. No. 3,398,259 discloses a copying electrical heating elements which, during a stand-by period in which fixing is not carried out, are connected in series in order to generate per unit of time a quantity of heat sufficient to keep the fixing device hot while, during a period in which fixing is carried out, only one of the groups is switched on in order to generate per unit of time a greater quantity of heat than during stand-by. The two groups can also be connected in parallel in order to generate per unit of time a quantity of heat which is even greater during a period in which the fixing device is heated up. The fixing device is provided with a temperature sensor which, when a temperature sufficient for fixing is reached, switches the fixing device over from the warm-up condition to the stand-by condition.
During stand-by periods, this fixing device will give up the most heat to the surroundings in those areas adjacent the surroundings, i.e. particularly the ends of the heating elements, and hence more in the edge zones than in the middle zone. During fixing, there is an extra heat yield to the image-bearing parts moving closely past the heating elements, such as the receiving support. This extra heat yield is substantially equal over the entire length of the heating elements In a copying machine of the kind in which a powder image is transferred from a photo-conductive support to a pre-heated receiving support via an image transfer medium in pressure contact both with the photoconductor and the receiving material, this extra heat yield transfer takes place to the photo-conductive support.
This copying machine and the method used therein have the disadvantage that the receiving support is heated nonuniformly in the direction transversely to the direction in which the receiving support is moved past the heating elements. Thus, it may readily happen that the temperature goes outside the temperature range in which good fixing is possible. Similar devices are also shown in U.S. Pat. No. 3,790,747 and Japanese Abstract No. 54-88134.
U.S. Pat. No. 4,301,359 discloses a fixing apparatus which always generates a fixed amount of heat per unit time, which amount is smaller in the middle zone of the heating element than in the edge zones thereof. Japanese Abstract No. 61-277986 discloses a device wherein the amount of heat generated per unit of time during fixing is less than that generated during stand-by. This device uses two temperature sensors which control two separate heating elements.
It should also be noted that U.S. Pat. No. 4,001,545 discloses a heat fixing device with a heating element comprising a first part and a second part both extending in the transverse direction, wherein in the first part the ratio between the amounts of heat generated per unit of time in a middle zone and in adjacent edge zones is less than said ratio in the second part. In this device, the amount of heat generated by the first part per unit of time is controlled via a temperature sensor in one of the edge zones, while the amount of heat that the second part generates per unit of time is controlled via a temperature sensor in the middle zone. Thus, this device requires two separate control circuits to control the temperature in the device thereby making the device unnecessarily complicated. A similar device is shown in Japanese Abstract No. 53-13432.
It would be desirable, therefore, to develop a method and device for fixing a powder image on a receiving support by means of heat which did not suffer from these disadvantages.